FIG. 1 schematically illustrates a cross-sectional view of conventional infrared color photographic film, such as Kodak Infrared Etachrome film. Upon a base A is superimposed three dye sensitized layers B, C and D. Each of the layers generally consists of an emulsion comprised of silver halide particles and a sensitizing dye. The sensitizing dye is a compound added to the emulsion layer to provide sensitivity to another portion of the spectrum in addition to the emulsion's natural sensitivity to blue light provided by the silver halide particles. In the prior art example of FIG. 1 layer B contains a dye sensitive to the infrared band, more particularly, the near infrared band with wavelengths of 700-1,000 nanometers. Layer C includes a dye sensitive to light in the visible red band whereas layer D contains a dye sensitive to the visible green band.
Infrared color film of this type is generally referred to as a false-color, reversal film and finds particular utility in aerial photography. It differs from ordinary color film in that the three sensitized layers are sensitive to green, red and infrared radiation instead of having the usual blue, green and red sensitivities. A yellow filter, such as the Kodak Wratten filter No. 12, is used on the camera lens to absorb the blue radiation to which all three layers are sensitive. When the film is processed, the green-sensitive layer D is developed to a yellow positive image; the red-sensitive layer C to a magenta positive image; and the infrared sensitive layer B to a cyan positive image. Since conventional film uses subtractive techniques, the green areas of the actual object actually are viewed as blue, the red comes out green, and the infrared regions are red when observing an image formed by the processed film. These techniques are well known in the art and the reader is referred to the technical literature for more details, see, e.g. Manual of Aerial Photography, Am. Soc. of Photogrammetry, 1st Ed. (1968).
One of the problems with conventional infrared color photographic film is that it is not as versatile as it could be. With the normally used Wratten 12 filter, conventional IR film is limited to providing information relating to the green, red and infrared bands. By filtering, some choice of just two of these bands may be provided. In any event, known IR films do not offer the option of responding to the blue light in only one of the sensitized layers. In other words, all of the layers in the prior art film are sensitive to blue light and thus, the developed film is not capable of providing information in the blue visible band. In some instances it would be highly advantageous to obtain information contained in the blue visible band. For example, the contrast between the blue visible band and the IR band is advantageous in detecting mine fields in aerial reconnaissance. Another instance is in taking pictures of objects at the bottom of bodies of water where the blue and green bands are useful for water penetration. Those skilled in the art will find other applications in which a more versatile infrared film may be used.